A two-component developer used in electrophotography is composed of a toner and a carrier, and the carrier is a carrying substance that is mixed with the toner under agitation in a developer box, provides a desired charge to the toner, carries the charged toner to an electrostatic latent image on a photoreceptor, and forms a toner image. The carrier is held by a magnet and is left on a developing roll even after forming the toner image and further returns to the developer box, and is again mixed and agitated with new toner particles, and repeatedly used for a certain period.
Since different from single-component developers, in the two-component developer, the carrier has functions to agitate toner particles, to provide desired charging properties to the toner particles, and to carry the toner; and has high controllability in the design of developers, it is widely used in the field of full-color machines wherein high image quality is required, and high-speed machines wherein the reliability of image preservation and durability are required.
In such a two-component electrophotographic developer, in order to obtain high-quality images, ferrite, such as Cu—Zn ferrite and Ni—Zn ferrite, is used as a carrier in place of oxide-film coated iron powder or resin-coated iron powder. Although these ferrite carriers have many advantageous characteristics for obtaining high-quality images compared with conventional iron-powder carriers, since the surface irregularities are uneven within the particles and between the particles, there is a problem of intense charge leakage particularly from the convex portions thereof, and high image quality is difficult to obtain.
For this reason, many attempts have been made to prevent charge leakage by imparting high resistance and high dielectric breakdown voltage to ferrite; however, these are insufficient. For example, in Japanese Patent Application Laid-Open No. 8-194338 or the like, a ferrite carrier containing an additive, such as zirconium oxide is disclosed, which is to control the surface properties, resistance and the like by adding the additive. However, it was difficult to disperse such an additive evenly in the ferrite, which promoted unevenness. When a large quantity of an additive was added to elicit its effect, for example, as described in “Electronic Material Series, Ferrite, p. 43, Table 3.2, Maruzen Co., Ltd.”, the growth of grains was suppressed, or on the contrary, the reaction was excessively promoted to loose evenness, or to produce other defects.
Thus, although various attempts have been made to impart high dielectric breakdown voltage to ferrite carriers, and thereby provide high-quality images, satisfactory effects have not been obtained as described above.